Certain types of radio frequency (RF) receivers that utilize a low noise amplifier (LNA) need to perform under conditions that demand very high linearity of the LNA and RF down conversion electronics. Such receivers are used in code division multiple access (CDMA) wireless communication systems. In a CDMA wireless communication system, users share all time and frequency resources simultaneously. This is accomplished by assigning each user a distinct Walsh code or user-unique digital code with special properties to achieve minimal interference between users. This code is added to the information data and modulated onto the carrier. An identical code is used in the receiver which is used to correlate the two signals. The correlation process only passes data that matches the code sequence. In this way, non-valid signals (i.e., signals from other users) are not decoded and appear as noise. Noise immunity is provided by means of the code spreading the spectrum of the information signal to a much wider bandwidth than is needed for baseband signal transmission. This ability of a CDMA system to reject unwanted signals allows for operation in high noise environments. Further information regarding CDMA systems is set forth in the well-known IS95 standard.
In one mode of operation, an IS95 compliant CDMA system permits duplex communications--that is, simultaneous transmission and reception of an RF signal. One problem of this mode is that the presence of an interference signal as the transmit (Tx) channel approaches its maximum power output generally causes cross-modulation of the Tx signal envelope. This results in "in-band" interference in the Rx channel. Such in-band interference can cause degradation in the received signal quality and subsequent call drop.
As is known in the art, the input third-order intercept point (IP3) of an LNA/mixer channel can be adjusted to a high value to keep cross-modulation within tolerable limits. The IP3 is a virtual measurement of the signal strength at which the power of the 3rd-order distortion energy of a gain stage is as strong as the fundamental signal energy. Using a high IP3 value increases the linearity of an LNA/mixer channel. However, a high IP3 value also results in a high bias current to the LNA/mixer channel, draining battery power and thus reducing both talk and standby time for an RF telephone. Nevertheless, maintaining a high IP3 value is the conventional approach to overcoming the cross-modulation problem noted above.
Accordingly, the inventor has determined that a better method and system is needed to control cross-modulation during duplex communications of an RF communication system that overcomes the problems noted above. The present invention provides such a method and system.